Contribution of exotic species, environmental factors and spatial components to the macrophyte assemblages in a Mediterranean lagoon (Thau lagoon, Southern France)

In the Thau lagoon, located on the French Mediterranean Coast, more than 20% of the macrophyte species are introduced species, likely through shellfish farming. Their impact on local soft substrate macrophyte communities has not yet been evaluated. In this study, we estimate the relationship between introduced species and the assemblage of indigenous macrophytes. Exotic species, environmental factors, and spatial distribution of macrophytes were analyzed with redundancy analysis (RDA) and partial redundancy analysis. We used data from a spatial survey that provided macrophyte densities, environmental characteristics (depth, granulometry, porosity, organic matter, phosphorous and nitrogen concentrations in the sediments), and geographical coordinates of sampling sites. The result of RDA showed that the macrophyte community structure can be explained by pure effects of exotic species (7%), pure effect of environmental factors (18%), pure spatial distribution (11%), and interactions among the above-mentioned variables (20%). Three exotic species (Dasya sessilis, Chrysymenia wrightii, and Griffithsia corallinoides) were found to have a significant effect on indigenous macrophytes distribution. C. wrightii co-occurring with Cladophora sp. and Radicilingua thysanorhizans, whereas G. corallinoides co-occurring with Cutleria multifida. Depth, porosity, total P and N and the granulometric fraction 0–2 μm were identified as the best environmental predictors. Polysiphonia elongata and Stictyosiphon adriaticus were found in the deepest sites, whereas Nanozostera noltii was correlated with shallow waters. The distribution of Cladosiphon mediterraneus, Gracilaria bursa-pastoris, and Dictyota dichotoma depended on freshwater inputs, whilst Rytiphlaea tinctoria and Halopithys incurva were related with marine water inflows.To further investigate the relationships between species and explanatory variables, logistic regression was used expressing the presence–absence of local species with variables identified during the RDA.